In
addition to heart rate, blood pressure, respiratory rate,
and temperature, pulse oximetry (PO) is considered to
be the “fifth vital sign” of health status. Hemoglobin (Hb)
is an essential part of the red blood cells responsible for
delivery of oxygen from the lungs to the tissues. The amount
of oxygen (O2) bound to Hb at any time is called
oxygen saturation. Expressed as a percentage, the oxygen
saturation (SpO2) is the ratio of the amount of oxygen bound
to the Hb to the oxygen carrying capacity of the Hb. Pulse
oximetry provides a noninvasive way to measure the SpO2, or
arterial hemoglobin saturation. Pulse oximetry also detects
arterial blood pulsations, and therefore also calculates and
reports a patient’s heart rate. A pulse oximeter is a
medical device that measures the amount of oxygen in a
patient’s arterial blood.

A
typical oximetry sensor has a pair of light-emitting diodes
(LEDs) facing a photodiode through a translucent part of the
patient’s body, usually a fingertip or an earlobe. One LED
is red, with wavelength of 660 nm; the other is infrared,
with a wavelength of 940 nm. The percentage of blood oxygen
is calculated based on the absorption rate from each
wavelength of light after it passes through the patient’s
body.

The
generic structure of the pulse oximeter is shown on Figure
1. In addition to analog signal conditioning circuitry, it
includes several ADCs and DACs, along with a Microcontroller
and associated memory. Implementations using this discrete
approach are relatively complex and costly in terms of
hardware, real estate, and design time.

Figure
1. Discrete Pulse Oximeter Implementation

The
Precision Analog Microcontroller family of products from
Analog Devices includes the key analog building blocks
required by a high-end oximetry design. The
ADuC7024, used here, includes a high-performance,
high-speed, multichannel 12-bit, 1-MSPS ADC and two DACs.
These features allow recovery of the weak arterial pulsatile
signals generally seen during low peripheral blood
circulation.

The
MicroConverter also includes a 32-bit ARM7TDMI core. Running
at 41.8 MHz, it provides a very powerful computational
platform. A proprietary digital signal processing algorithm,
developed by ITEC Engineering, detects arterial blood
pulsations, detects and rejects motion artifacts, calculates
SpO2 and heart rate values, and filters and scales the real
time arterial blood pulsatile waveform (plethysmogram).
Still, plenty of CPU performance is left for additional
functions, such as control of the graphics LCD display,
generating audio pulse tones via the onboard PWM, etc.

The
structure of the oximeter based on the ADuC7024
MicroConverter is shown on Figure 2:

Figure
2. Pulse Oximeter Implementation Using ADuC7024
MicroConverter

The LEDs are powered sequentially through a MOSFET bridge (switches SW1 and
SW2).
The constant current sink , built with AMP1 (1/2
AD8606), is controlled by the DAC2 output of the
MicroConverter.

The
photodiode amplifier is built around AMP2 (1/2 AD8606). It
converts received light into a voltage that is inversely
proportional to the light absorbed by the patient’s tissue.
This measurement is made on red and infrared wavelengths
alternatively.

The
programmable-gain amplifier (PGA) and offset stage is built
with an AD8606 opamp and an
AD5160 digital potentiometer. It is dynamically
controlled by the MicroConverter. Together with the variable
current LED drive (AMP1 and DAC2), it provides for a wide
range of optical densities seen by the oximetry sensor.

Switch
SW3 (ADG779) removes the major portion of ambient light seen
by the photodiode, thus allowing the full dynamic range of
the ADC input channel to be utilized.

The
input of ADC1 is driven by the PGA/offset circuit. This ADC
digitizes the amplified photodiode signal. The output of the
photo amplifier AMP2 is ac-coupled, so ADC2 monitors any
possible ambient light saturation of AMP2.

This
design could be used as a stand-alone oximeter or as a
building block for various medical monitoring devices.

The
ADuC7024 MicroConverter, with 30 general-purpose I/Os (GPIOs),
was selected for this design due to the high number of I/O
pins required for interfacing with the LCD. With fewer I/Os,
the same level of performance could be achieved by using the
ADuC7021 (13 GPIOs, no PWM). The ADuC7021 is available
in a space saving 6-mm × 6-mm LFCSP package.